Thermal barrier coatings (TBC) are generally applied onto metallic surfaces of, for instance, gas turbine engines or aero-space engine parts. Since such engines operate at highly elevated temperatures, the surfaces of the metallic substrates tend to thermally expand. This may cause structural defects in engines during operation and further operating failure. Thus, thermal barrier coatings are essential for those engines in order to protect the surface from thermal stress and prevent the surface damage.
However, thermal barrier coatings often fail as a result of coating buckling, peeling, detaching, or even spallation during operation. These conditions may be caused by many factors including thermal stress and environmental stress.
During the operation of such aero-space substrates, thermal barrier coatings are continuously exposed to environmental contaminants, such as a dust, sand, ash, or small debris. Among those, calcium-magnesium alumino-silicate (CMAS) contaminants cause significant damages to thermal barrier coatings. For example, at operating temperatures exceeding 1200° C., CMAS debris melts and deposits on the surface. Occasionally, such molten CMAS penetrates the thermal barrier coatings and then solidifies, ultimately causing spallation and destruction of coating.
In order to protect thermal barrier coatings from the CMAS contaminants, several methods have previously been employed. For example, an additional protective layer has been previously applied on the surface of thermal barrier coating (e.g., U.S. Pat. No. 8,470,460; US. Pub. No. 2013/0260132). Similarly, rare earth elements have previously been added to the coating components for CMAS resistance (e.g., U.S. Pat. No. 6,284,323; US. Pub. No. 2011/143043). The rare earth element gadolinium is often used in thermal barrier coatings to improve CMAS resistance. Gadolinia (gadolinium oxide, Gd2O3) is the most commonly available form of gadolinium. Gadolinia reacts with CMAS to increase the viscosity and the melting point of CMAS. As result, CMAS contaminants do not infiltrate thermal barrier coatings as deeply at the operating temperature, reducing the amount of damage caused by CMAS in thermal barrier coating.
However, high gadolinia content in thermal barrier coatings has caused reduction in durability and spallation resistance of thermal barrier coatings. In addition, raw gadolinia is an expensive material for conventional thermal barrier coatings. Thus, balance between advantageous utilization and reasonable cost is critical for producing a CMAS resistant thermal barrier coating.
The present invention addresses the economical use of gadolinia for a CMAS resistant thermal barrier coating while maximizing durability to the thermally and mechanically induced stresses.